An Exercise in Daftness.

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SteveTheShadow
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An Exercise in Daftness.

Unread post by SteveTheShadow » Sun Nov 11, 2018 4:16 pm

The output transformerless valve amplifier has to be one of the daftest amplifying contraptions ever; woefully inefficient and more useful as a room heater than actually amplifying your music signal, yet possessed of a sound so clear, sweet and musical, that once heard, it is never forgotten.

Now I've built four myself, two of which promised a lot, but were plagued with problems; so much so that they became more trouble than they were worth and ended up in my electronics graveyard. The other two were well-proven designs by Bruce Rozenblit of Transcendent Sound, in the USA, built from kits. I built up these for Nigel, one of our Ant's customers. Both worked first time without issue. One; an OTL that uses eight 300Bs (four per channel) has been with Nigel for around 12 months. The other is a single ended OTL, using six (three per channel) EL509 colour TV line output pentodes, or "sweep tubes" as they are known to our American friends. This OTL amp is waiting to be picked up by Nigel, along with a Goldring G99 idler turntable that Ant has serviced and replinthed for him. Testing this latter OTL on my Fane retro speakers has produced some extremely fine music and I'll be sorry to see it go.

So I've had a look in my valve boxes and fished out a pair of rather large DC control valves, that are Ideal for use in an output transformerless amplifier, because they have low output resistance and high transconductance, and have drawn up a circuit for single ended operation, on the back of a fag packet:


It's a three stage job with a JJ ECC99 double triode acting as input and driver stage.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Large sealed cabs, with Fane 12-250-TC 12" full range drivers.

SteveTheShadow
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Joined: Thu Sep 12, 2013 5:24 pm
Location: S Yorks
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Re: An Exercise in Daftness.

Unread post by SteveTheShadow » Sun Nov 11, 2018 4:30 pm

Below is the layout:
Image

It'll pull about 200W out of the wall and put out 2Wpc. 1% efficient :lol:
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Large sealed cabs, with Fane 12-250-TC 12" full range drivers.

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Firebug1
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Joined: Thu Nov 20, 2014 5:07 am
Finland

Re: An Exercise in Daftness.

Unread post by Firebug1 » Sun Nov 11, 2018 5:13 pm

Hopefully the heat it developes keeps you warm in coming winter time :grin:
Rega\Teac\NVA\SA Mantra30\ATH-A900X
...the Gods you worship are steel, at the altar of rock 'n' roll you kneel...

keepitsimplestupid
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Joined: Mon Feb 26, 2018 2:18 pm
Great Britain

Re: An Exercise in Daftness.

Unread post by keepitsimplestupid » Sun Nov 11, 2018 7:11 pm

Lovely looking bit of stupidity, 1% efficiency. What's not to love about that.

SteveTheShadow
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Location: S Yorks
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Re: An Exercise in Daftness.

Unread post by SteveTheShadow » Sun Nov 11, 2018 11:36 pm

Here's the schematic:
Image

TL783 regulator in the cathode of the output stage, is wired as a constant current sink and clamps the current through the tube to 520mA courtesy of the 2.4 ohm resistor between the "out" pin and ground. These kinds of big, high transconductance, industrial power management valves will overheat at the slightest opportunity, if used as audio tubes, so a bit of silicon in the tail keeps the current tightly stabilised. That, in theory will stop any attempt to overheat, dead in its tracks.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Large sealed cabs, with Fane 12-250-TC 12" full range drivers.

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Thermionic Idler
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Great Britain

Re: An Exercise in Daftness.

Unread post by Thermionic Idler » Mon Nov 12, 2018 7:00 pm

Damn shame that Fane don't make 15 ohm versions of that full range driver.

SteveTheShadow
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Re: An Exercise in Daftness.

Unread post by SteveTheShadow » Mon Nov 12, 2018 7:25 pm

Thermionic Idler wrote:
Mon Nov 12, 2018 7:00 pm
Damn shame that Fane don't make 15 ohm versions of that full range driver.
Yes, a pity that :cry:
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Large sealed cabs, with Fane 12-250-TC 12" full range drivers.

SteveTheShadow
Posts: 1042
Joined: Thu Sep 12, 2013 5:24 pm
Location: S Yorks
Great Britain

Re: An Exercise in Daftness.

Unread post by SteveTheShadow » Sat Nov 24, 2018 4:05 pm

OK, so in the spirit of chainsaw hi-fi, the transformers are mounted on the bit of ply and everything has been tested.

Image

The correct AC off-load voltages are appearing where they should and the CL60 inrush current limiting thermistor is doing a fine job of stopping the breaker tripping when the 1200VA's worth of toroids are all fired up at the same time.

Centre is the 1KVA HT transformer for the output stage.

One 115V winding for the output stage, the other will feed a full-wave voltage doubler, to give 300V DC, for the input and drivers.
I'm doing this because I have no suitable transformers that'll give the right HT for the input driver cascade.
Can't see the doubler being a problem for this low-current, input/driver application.

Left is the heater transformer for the 13E1s
Right is the heater transformer for the input and driver stages.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Large sealed cabs, with Fane 12-250-TC 12" full range drivers.

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Dr Bunsen Honeydew
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Re: An Exercise in Daftness.

Unread post by Dr Bunsen Honeydew » Sat Nov 24, 2018 4:20 pm

:mrgreen: Everyone need a hammer when building amps

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karatestu
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Re: An Exercise in Daftness.

Unread post by karatestu » Sat Nov 24, 2018 4:34 pm

SteveTheShadow wrote:
Sat Nov 24, 2018 4:05 pm
OK, so in the spirit of chainsaw hi-fi, the transformers are mounted on the bit of ply and everything has been tested.

Image

The correct AC off-load voltages are appearing where they should and the CL60 inrush current limiting thermistor is doing a fine job of stopping the breaker tripping when the 1200VA's worth of toroids are all fired up at the same time.

Centre is the 1KVA HT transformer for the output stage.

One 115V winding for the output stage, the other will feed a full-wave voltage doubler, to give 300V DC, for the input and drivers.
I'm doing this because I have no suitable transformers that'll give the right HT for the input driver cascade.
Can't see the doubler being a problem for this low-current, input/driver application.

Left is the heater transformer for the 13E1s
Right is the heater transformer for the input and driver stages.
Hey, are you copying my design ideas Steve ? :lol: :lol: :lol:
Dr Bunsen Honeydew wrote:
Sat Nov 24, 2018 4:20 pm
:mrgreen: Everyone need a hammer when building amps
The hammer isn't big enough Doc :lol: :lol: :lol:

Naim used to hammer their toroids to get them flat enough to fit in the cases ya know :lol: :lol: :lol:
Naim, Linn & Harbeth fanboy

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